Gas-liquid contacting is considered to be one of the most important unit operations in several sectors of industry. Such contacting may substantially determine the feasibility of the whole technology as well as the technical parameters of the products.
The efficiency of gas-liquid contacting has a decisive role in most of the aerobic processes in the fermentation industry, in the aerobic biological purification of sewage as well as in a number of chemical processes.
The known gas-liquid contacting systems can be grouped according to the method of energy transfer as follows:
pneumatic systems (bubble columns, air-lift loop reactors etc.)
mechanical systems (surface aerators with horizontal or vertical shaft, self-sucking stirrers)
combination of the above systems (gas-sparged stirred reactors)
hydraulic systems.
As far as the efficiency of the energy transfer is concerned, hydraulic systems proved to be the most advantageous techniques in gas-liquid contacting, manifested in the increasing spread of this method in the last years.
A common characteristic of the hydraulic systems is that the gas-liquid contacting is carried out by liquid jets of various forms produced by a pump and some kind of a nozzle.
Depending on the character of the liquid jet, these processes can be distinguished as follows:
processes using disrupted liquid jets (spraying towers, Venturi scrubbers)
processes using two-phase liquid jets (injectors and ejectors)
processes using homogeneous, coherent, plunging liquid jets.
Within the hydraulic systems these latter type of processes can provide both the most advantageous energy efficiency and the highest possible specific mass transfer rate (intensity of gas-liquid contacting) as well as the lowest specific investment costs.
A common feature of the plunging liquid jet processes is that the homogeneous, coherent liquid jet, issued from the nozzle above the surface of the liquid body, travels through the gas space above the liquid surface and enters the bulk of the liquid while entraining a large amount of the gas from the gas space above the liquid surface. The entrainment of the gas is carried out in such a way that--due to the surface roughness of the liquid jet--a gas boundary layer is being developed on the surface of the jet while it passes through the gas space and, entering the liquid body together with the liquid jet itself, it is broken up into fine bubbles under the effect of shear forces between the jet and the liquid body.
The efficiency of these processes is simultaneously determined by the surface roughness and the coherency of the liquid jet in the following way:
the greater is the surface roughness of the liquid jet, the higher can be the gas entrainment rate, thus the quantity of the gas to be dissolved will be increased
the more coherent the liquid jet is, the finer gas dispersion and the deeper bubble penetration depth can be achieved (the longer will be the residence time of the bubbles), thus the intensity of contacting will be increased.
Generally, it can be stated that none of the known plunging jet gas-liquid contactors can satisfy simultaneously and advantageously the above-mentioned two requirements, i.e. the known techniques can increase the surface roughness of the jet only by simultaneously diminishing the coherency of the liquid jet or vice versa.
To increase the surface roughness of the liquid jet one or the combination of the following methods is used without exception by all of the known processes (e.g. Chem. Eng. Sci. 36, 1161 /1981/; Chem. Eng. Commun. 15, 367 /1982/; published Hungarian patent application No. 3901/81):
using a nozzle having a shape differing from the hydraulic optimum
increasing the velocity of the liquid jet
increasing the level of turbulence of the liquid jet
increasing the free length of the liquid jet.
The common disadvantage of these methods is that, on the one hand, they cause significant hydraulic losses, hence decreasing the energy efficiency of contacting, and, on the other hand, all of these methods result in decreasing the coherency of the jet, hence decreasing the intensity of contacting.